The present invention relates to a control apparatus and a control method for a multi-cylinder engine and, more particularly, to a control apparatus and a control method for a multi-cylinder engine that suppresses, in a favorable manner, variations in an ignition combustion state among different cylinders during an engine operation in a compression ignition combustion mode.
There is known a multi-cylinder engine that works on a combustion principle, in which a premixture is compressed and self-ignited. This type of engine is, what is called, a homogeneous charge compression ignition combustion multi-cylinder engine. It is desired that this type of engine be controlled in a favorable manner so that it performs a self-ignition combustion that exhibits high efficiency over a wide operating range. For example, Japanese Patent Laid-open No. 2000-320333 discloses a technique, in which valve timing of intake and exhaust valves are controlled so as to provide a valve shutoff period (minus overlap) in the cylinder. This minus overlap period and fuel injection timing are controlled in accordance with changing operating conditions, thereby controlling self-ignition combustion of a mixture.
The technique disclosed in the aforementioned publication is intended to control various components of the engine according to engine operating conditions. If, for example, there are part-to-part variations among those engine components or if any deteriorates, however, a symptom could occur, in which an ignition timing and combustion state vary from one cylinder to another and from one cycle to another even under the same operating conditions. This symptom could cause pre-ignition or misfire to occur in one or more cylinders of the engine. There is as a result a problem arising, in which fuel consumption and the state of exhaust emissions are seriously aggravated.
In a commonly known spark ignition multi-cylinder (gasoline) engine, control is provided with the ignition timing of an ignition plug first established. In a diesel engine, too, combustion control is provided with the ignition timing for fuel spray first established. Both are equipped with an ignition trigger and capable of an extremely powerful and highly robust combustion control. There has therefore been little necessity to identify detailed combustion states for each cylinder and provide combustion control based the combustion states.
With the homogeneous charge compression ignition combustion multi-cylinder engine, however, it is assumed that a premixture of air and fuel self-ignites as a piston compresses it. For that reason, this type of engine is not provided with any powerful ignition trigger as in the spark ignition engine or the like described earlier and a combustion period thereof is shorter than that of the spark ignition type or the like. This very fact enables operation at high efficiency with low exhaust emissions. At the same time, even the slightest variations in the cylinder structure greatly affects a combustion condition, which eventually aggravates fuel consumption and the state of exhaust emissions. This gives a good reason to believe that there is a cylinder or two, in which combustion does not occur normally, even if each of all cylinders is controlled in exactly the same manner. It is ascertained through experiments conducted by the applicant.